GitLab

TestConnection now reuses antigravityRetryLoop instead of a standalone
HTTP loop, gaining credits overages, smart retry, and 429/503 backoff
for free. AccountSwitchError is caught and surfaced as a friendly
message. Also populates RateLimitedModel in TempUnscheduled switch error.

Test fixes:
- Use RATE_LIMIT_EXCEEDED in 503 short-delay test to avoid 60x1s timeout
- Clamp waitDuration=0 instead of 999s to avoid 15s max-wait timeout
- Enhance mockSmartRetryUpstream with repeatLast and body caching